Computed tomography systems typically include a radiation source which transmits an electron beam towards and onto an anode to define a focal spot from which the radiation is emitted. One or more beam collimators defines a fan-shaped beam emanating from the focal spot, and a bank of x-ray detectors located opposite the radiation source receives the x-ray beam. In third-generation computed tomography systems, the radiation source and the x-ray detectors are both mounted on a rotatable gantry for rotation around an object to be scanned.
The anode is subject to overheating from impingement of the electron beam on it. It is thus designed to rotate rapidly so that the electron beam does not strike the anode in any single location for more than a fraction of a second, thereby reducing the risk of localized overheating and possible melting of a portion of the anode.
Notwithstanding the rotation of the anode, heat from the electron beam causes the anode and its support structure to expand, and this causes the focal spot on the anode to drift. The effects of gravity and rotation of the radiation source also contribute to focal spot drift. Such focal spot drift typically occurs in the axial, or z, direction and causes the fan beam to change its position on the primary x-ray detectors, also in the z direction. Changes in beam position on the primary detectors may produce variations in the gain and energy sensitivity of the detectors, particularly if the detectors are calibrated and then not used until some time (days or weeks) later. This can cause ring artifacts to appear in the reconstructed image.
It is known to correct or compensate for focal spot drift by, for example, using x-ray detectors which are highly uniform in the z direction, and thus insensitive to beam motion in the z direction. However, solid-state detectors having this characteristic are very expensive to manufacture. Gas detectors are a less expensive alternative. Although xenon gas detectors are more uniformly sensitive than solid-state detectors, they are significantly less efficient and are therefore not a desirable alternative.
Another method of correcting focal spot drift is to use a post-patient collimator positioned between the object being scanned and the detectors. The post-patient collimator is preferably located as close to the detectors as is practical and restricts the size of the beam so that the beam that reaches the detectors is smaller than the beam that reaches the patient. As the beam moves in the z direction, the edges of the beam are masked by the post-patient collimator and never impinge on the detectors. The collimated beam impinging on the detectors can therefore be maintained in a fixed position relative to the detectors.
A disadvantage of this technique is that a potentially significant amount of radiation may pass through the patient without being detected. The patient is thus exposed to radiation in excess of that which is used to provide diagnostic information.
In the prior art, the beam position on the x-ray detectors may be sensed by a reference detector located, for example, at a peripheral edge of the beam. The portion of the beam directed to a peripherally-located reference detector is usually not occluded or shadowed by any object in its path. Thus, the magnitude of the signals from the reference detector should always be a constant value, except when the reference detector is obstructed or occluded. This approach is disclosed in, for example, U.S. Pat. Nos. 4,559,639 to Glover et al., 5,550,889 to Gard et al., 5,706,326 to Gard, 5,299,250 to Stymol et al., 5,131,021 to Gard et al., 5,065,420 to Levene, 4,991,189 to Boomgaarden et al., and 4,769,827 to Uno et al.
A disadvantage to the use of one or more reference detectors at a peripheral edge of the fan beam is that they are occasionally occluded, or shadowed, by the patient. If this happens, the reference detector will provide signals of varying, not constant, magnitude, which could indicate either that the beam has changed its position on the primary detectors, or the variation in signal magnitude is caused by shadowing of the reference detector. The reference detector will thus be unreliable for providing a signal of constant magnitude and thus cannot be used to confirm a constant beam position on the primary detector array.
To avoid the problem of a shadowed peripherally located reference detector, the reference detector can instead be located so that the portion of the x-ray beam reaching the reference detector is never occluded, such as, for example, between a beam-defining precollimator and the x-ray tube. In this approach, a separate, secondary beam is directed from the focal spot towards a secondary detector which may be out of the plane of, or beyond the edges of, the primary beam. Movement of the secondary beam relative to the reference detector indicates drift of the focal spot. A desired beam position on the primary detectors can be monitored and maintained in response to a signalfrom the reference detector, which drives a beam-defining collimator to place the beam in a desired position. This technique is disclosed in, for example, U.S. Pat. No. 5,550,886 to Dobbs et al., hereby incorporated by reference, as well as U.S. Pat. Nos. 5,469,429 to Yamazaki et al. and 4,803,711 to Tsujii et al.
The reference detector must be calibrated to achieve a desired primary beam position as a function of focal spot position. The long-term stability of the reference detector is a function of the stability of its calibration.
The prior art addresses the problem of long-term thermal drift of the focal spot and other system components, but not the problem of gravity-induced focal spot drift, which is a sinusoidal variation which occurs over the course of a single rotation of the system components.
Thus, there is a need for a computed tomography scanner which can provide a stable beam position on the primary detectors over both the long and short term, i.e., during many rotations of the source and detectors in a period which may span days, weeks, months or even longer, as well as during a single rotation of the source and detectors.